利用赤泥制备钙铝榴石微晶玻璃的实验研究

在1300℃下将赤泥和石英砂熔制成基础玻璃,经粉磨、筛选后,通过二次热处理工艺获得了以钙铝榴石为主晶相的微晶玻璃。采用XRD和SEM等研究赤泥和微晶玻璃后表明,赤泥主要由方解石和霰石等钙质成分组成;热处理温度对微晶玻璃性能和钙铝榴石晶体结构的影响较大。优化的制备条件为:核化900℃,1h,晶化1105℃,2h,该条件下所得的微晶玻璃晶相含量较高,晶体发育完整,性能良好,相应的体积密度为2.79g/cm3,显微硬度为6.52。     

一株岩生真菌对方解石的风化作用

选用一株从碳酸盐岩表面分离的真菌,研究了实验条件下该真菌对方解石的风化作用.通过检测风化培养基与对照培养基的pH值、Ca2+浓度、挥发性代谢产物以及分析真菌一矿物相互作用过程中的矿物表面微观形态变化,研究该真菌对方解石的风化过程和作用机理.结果表明:在实验条件下该真菌对方解石具有较强的风化作用,风化培养基上清液中的Ca2+浓度分别是死菌对照的3.98倍和纯水对照的13.73倍;该真菌生长过程中会分泌有机酸和醇类等代谢产物,真菌和矿粉之间会形成菌体-矿物聚集体或复合体,促进方解石的风化.分析认为该真菌对方解石的风化作用包括机械破坏,代谢产物溶蚀、络合作用以及多种因素之间的协同作用.     

卵磷脂与双甘氨肽对碳酸钙变体和表面微结构的影响

许多生物矿化作用过程都与其不寻常的膜的组成和结构密切相关。作为双亲分子的磷脂,是生物膜的重要组成成分之一。运用以粉末X-射线衍射、扫描电子显微镜和动态光散射技术等实验手段,研究了卵磷脂(PC)和双甘氨肽参与下的碳酸钙矿化过程。结果表明,PC在溶液中形成的脂质体结构,导致了方解石(calcite)表面多孔结构的形成,且其含量的改变,可有效地调控方解石表面孔隙的孔径大小,并在一定程度上促进球霰石(vaterite)的生长和稳定;而PC在溶液表面形成的膜脂层,则通过静电力与双甘氨肽相互作用,有效调控膜脂层下矿物的形成。     

黄河流域内蒙古段砒砂岩风化土微生物矿化改良的试验研究

砒砂岩,一种在黄河中上游广泛分布的特殊岩石,是由砂页岩和泥质砂岩组成的岩石互层,由于成岩程度低、沙粒间胶结程度差、结构强度低,且含有大量黏土矿物,其抗侵蚀能力弱,遇风成沙、遇水成泥,是“泥沙入黄”的重要来源。基于微生物诱导碳酸钙沉积(microbially induced carbonate precipitation,MICP)技术对砒砂岩风化土进行改良加固,以矿化后试样获得良好的强度为目标,结合物性及孔隙结构分析,对含有大量细粒土的砒砂岩风化土进行微生物矿化改良试验的最优方案设计。试验方案中设置12种工况,通过菌液浓度、菌液与胶结液用量比、钙尿摩尔比3个控制要素,对经微生物诱导沉积的碳酸钙晶体的晶型、形貌和尺寸进行人为调控。试验结果表明,当微生物矿化试验中尿素消耗量为0.4 mol时,采用菌液浓度OD₆₀₀值为1.2、菌液与胶结液用量比为1:20、钙尿摩尔比为1:1的试验方案,经微生物诱导的碳酸钙晶体以20～30μm的“方解石-球霰石团聚体”的晶型被沉积,并填充于砒砂岩风化土的孔隙中,使得砒砂岩风化土密实度提高,矿化后试样的孔隙度减小了62.4%,抗蚀能力得到...     

湘南多金属矿床方解石形态及表面微形貌的研究

对湘南多金属成矿区方解石的研究表明，产于钨矿床中的方解石以层状晶体发育为特征，晶体形态主要为｛０００１｝和｛０１２｝＋｛１０１０｝，晶体表面具有菱形生长阶梯；产于铅锌矿床中的方解石一般不出现层解石，晶体形态主要为｛２１３１｝，｛０１１２｝＋｛１０１０｝和｛０００１｝＋｛１０１０｝＋｛０１１２｝，晶体表面平行纹发育；远矿正常灰岩内方解石晶体形态主要为｛４０４１｝。     

洛川黄土纳米尺度观察：纳米棒状方解石

本文对洛川黄土剖面典型样品进行了透射电镜分析，发现黄土中存在纳米棒状方解石。此类方解石直径30～50nm，长度几百纳米至几微米。纳米棒状晶体有圆滑地粗细变化和弯曲现象，偶见晶体连接成树枝状或晶体直角连接形成框架状形态。X-射线能谱分析显示方解石有少量的镁、磷和硫，这种纳米棒状方解石在国内外文献中鲜有报道。根据纳米尺度研究结果推测，这种纳米棒状方解石形成与生物衍生物诱导的定向结晶有关。纳米棒状方解石是一种重要的黄土堆积时期干旱环境指示矿物，其碳、氧稳定同位素组成代表黄土堆积时气候参数。纳米棒状方解石的发现，对环境中纳米矿物研究、黄土中碳酸盐碳成因及古气候研究具有重要价值。     

土壤、大气尘中晶体的组成及其物源指示意义——以长沙、株洲和湘潭地区为例

基于长沙、株洲和湘潭地区大气尘(可吸入颗粒和近地表积尘)和表层土壤的X-射线衍射(XRD)分析数据,采用微量相鉴定方法获得的结果显示,大气尘和表层土壤中包含石英、石膏、方解石、白云石、硫酸镉等自然矿物及SiS_2、NiTi、Cd_3As_2、FeOCl等非自然源晶体。比较了长沙、株洲、湘潭地区大气尘与表层土壤的晶体组成,分析了大气尘矿物及非自然源晶体的空间分布,探讨了其环境指示意义。     

松辽盆地古龙凹陷白垩系青山口组储集空间与油态研究

松辽盆地古龙凹陷有丰富的页岩油气资源。但是目前对古龙凹陷页岩油储层的岩石学特征和页岩油的状态,尤其是储集空间的类型及其组合关联性认识不清。通过对古龙凹陷白垩系青山口组岩芯的精细描述及其薄片分析和电子显微镜观察及三维CT深入研究,发现古龙青山口组页岩油的储集空间具有多样性和多尺度性。除了纳米孔缝外,还有页理缝。根据页理缝的规模和与油气的关系,可分为5类：① 纳米缝,缝宽在10~50 nm,缝长50~100 nm,或更长;两端尖中间宽,微弯曲呈蠕虫状;本身也是重要的储集空间,是纳米级油元的重要组成部分,与纳米级孔油元和微米级油基关系密切;② 微米缝,宽 0.1 μm 到数微米,长数十微米到数百微米,与纳米级油元和微米级油基关系密切;③中微缝,宽数微米到数十余微米,缝长数百微米,与微米级油基关系密切;④大微缝,宽数十微米到100 μm,长数百微米到数毫米,与微米级油基和微微缝及中微缝关系密切和⑤ 大页理缝(宽数百微米,肉眼明显可见),与各级微裂缝关系密切。此外,可见高角度倾斜或直立的裂缝,由于这些页理缝顺层发育,所以往往当做页理对待。通过研究,认为页理缝主要是嫩江组沉积末期（嫩末）和明水组沉积末期（明末）的构造反转褶皱过程中形成的。另外,还发育了大量的顺层方解石脉,根据方解石脉的宽度分为3类：① 小型介于0.1~1.0 mm;② 中型介于1.0~5.0 mm;③ 大者介于0.5~1.0 cm,最大宽度2.5 cm。较大的顺层方解石脉由垂直页理的纤柱状方解石组成。大方解石普遍发育共轭裂缝和挤出构造,是古龙凹陷嫩末和明末古应力反转的结果,也是古应力恢复的重要依据。经应力恢复认为嫩末和明末,可能一直延续到依安组的最大应力来自于水平方向（东西方向）,在1500 m和2500 m深度水平最大挤压应力分别可达139.16 MPa和204.27 MPa,而垂向最小应力则分别为35.44 MPa和59.07 MPa。所以,在这种应力状态下导致顺层发育了大量页理缝和顺层纤柱状方解石脉。此外,在页理面上还发育了一系列摩擦镜面、擦痕、阶步、光面、剪裂面、鳞片构造、碎片构造等,揭示了沿页理发育了强烈的顺层剪切。四级纳微缝与大页理缝密集发育,在顺层面方向构成了裂缝空间联通网络,使页岩在顺层面方向渗透率较好或很好;裂缝空间联通网络与纳米和微米孔一起构成了一个三维的特殊缝孔体,与碳酸盐岩的缝洞体相当。纳米孔缝和微米孔缝及页理缝对于松辽盆地青一段页岩油的勘探开发具有重要意义。     

多组分有机质作用下碳酸钙的矿化现象

采用SEM、XRD、FT-IR等手段对氨基酸、多糖以及有机酸混合体系中形成的碳酸钙晶体进行了表征,进而比较了多组分有机质作用下碳酸钙的矿化现象。结果表明,氨基酸-单糖体系(A-M)促进了碳酸钙的沉积,其钙化速率相较于对照组提高了36%;有机酸则抑制了碳酸钙的沉积,单糖-有机酸体系(M-O)、氨基酸-有机酸体系(A-O)和氨基酸-有机酸-单糖体系(A-O-M)的钙化速率分别下降了33%、29%、17%。其次,A-M体系合成的晶体以方解石为主,包含少量的球霰石,M-O、A-O和A-O-M体系则在柠檬酸的调控下合成了方解石。最后,A-M体系合成了块状晶体(粒径为10μm)及中空的环状晶体(粒径为5μm),而A-O和M-O体系合成了棒状晶体(长轴径5～15μm,短轴径3～5μm)。A-O-M体系存在含量最高的有机质,导致成核速率高于生长速率,因此合成的晶体呈粒状(粒径1～3μm)且发育不完整。研究揭示了多组分共存体系中的生物矿化现象,阐明了柠檬酸在多组分体系中对晶体成核及生长的主导调控作用,对研究黄龙高寒环境下的生物矿化机制具有重要意义。     

浅谈观赏石的分类及评价原则

观赏石或称玩石、奇石、珍石、雅石和欣赏石,属宝石家族成员,是天然形成的有欣赏价值和商品价值的矿物岩石艺术品,又称艺石。它包括矿物晶体、造型石、纹理石、工艺石、生物化石、文房石和盆景石等。它以奇特的造型、美丽的色彩及花纹、细腻的质地而深受人们的喜爱。它们“物以稀为贵”,它又以天然形成为主,一般不作加工,注重形态、纹理,兼顾色彩、质地及产量,主要用于陈列、收藏及造园等。观赏石大致分为三个类型。一类是矿物晶体型,即强调矿物的颜色及组合晶体的完整性和晶体的名贵性。首先是名贵矿物晶体,如钢灰色放射状辉锑矿晶簇,黄色放射状锑赭石晶簇,赤如鸡冠短柱状雄黄与方解石晶簇,柠檬黄或金黄色放射状雌黄与方解石晶簇,多色彩长柱状电气石水晶晶簇,海蓝色柱状海蓝宝石与水晶晶簇,鲜红色箭簇状穿插双晶辰砂与方解石白云石晶簇,钢灰色柱状针状放射状辉铋矿晶簇,等等。其次是矿物以色、形或组合惊人,即矿物本身并不名贵,主要     

Pyritized mudstone and associated facies in the Permian–Triassic boundary of the Çürük Dağ section,Southern Turkey

This paper is the first study of pyritized mudstones (PM) in the Permian–Triassic (P–T) boundary section of the Çürük Da? (Taurus, Antalya Nappes, Turkey). The mudstones were generally formed as lensoidal-shaped layers or infill materials within nodular platform limestones (hardground). Normal marine fauna is diminished in the pyritized limestones, whereas tube-like microorganisms are apparently increased with the association of pyrite crystals consisting of both framboidal and cubic crystals. The total rock volumes are up to 50–60% clay minerals and are mainly made up of in situ kaolinite and subordinate mixed layer clays (illite–vermiculite). Kaolinite preferentially developed on feldspar crystals, sometimes covering ostracoda bivalves together with gypsum micronodules composed of fan-shaped gypsum crystals. The origin of the kaolinite is, in situ, directly related to feldspar dissolution via heterotrophic bacteria. Thus, kaolinite is found along with bacterial structures. Other mineralogical compositions include established quartz (mostly β-quartz), gypsum crystals (100–200 μm) glauconite and magnetite. Magnetite grains comprise a minor amount (1–2%) and show some bacterial-induced crystal orientations. Glauconite is formed as an accessory mineral that occurs as infill material in biogenic grains. On the other hand, some microspheres represented by a silica-dominated composition are only observed in scanning electron microscopes (SEM) studies under high magnification. Isotope values (d34S) obtained from the pyritized mudstones show an isotopic heterogeneity that suggests that the pyritized mudstone consists of at least two components, with different sulphur-concentrations and d34S values.     

Discovery,Genesis and Significance of Metallic Minerals in Evaporate Series in the Kuqa Basin

A large-scale evaporate series is developed in Paleogene-Neogene strata in the Kuqa basin. The series is composed mainly of evaporate with thin beds of clastic rock (mainly mudstone and siltstone). In grayish white medium- and coarse-grained sandstone in Miocene strata, the formation of copper minerals is in close connection with brine. In joint planes, which are developed in vertical strata, are filled with gypsum. Gypsum and copper-mineralized sandstone contains enormous copper minerals, mainly atacamite. According to the SEM analysis for salt rock, gypsum rock, limestone, grayish green siltstone, grayish white medium-coarse-grained sandstone, some minerals are composed of metallic elements including Au, Ag, Cu, Zn, Pb, Co, Ni and U etc., in which Au occurs in a native form, Cu occurs in a native form or as atacamite in salt rock, gypsum rock and limestone, Ag occurs as silver sulfide in gypsum, and Zn, Pb, Co, Ni, U occur as compounds along with the above metallic ions in evaporate or clastic rock. From SEM images, we can see that metallic elements or their compounds (oxides or sulfides) “take root” as grains in salt or gypsum crystals, which belong to primary chemical sedimentation along with evaporate, while some grains “float” on surface of salt or gypsum. In the former case, mineral grains were formed together with salt (gypsum) crystals; while in the latter case, minerals were enriched from internal metallic ions (Paleogene evaporate samples) or external metallic ions (Neogene gypsum samples) in the late stage of evaporate formation. The metallic ions in Paleogene evaporate samples might originate from weathered or denudated materials in the south Tianshan Mountains. The metallic ions in the Neogene evaporate samples might be from metal-bearing brine, which migrated upward to surface along fractures and leached into evaporate (gypsum). Occurrence of metallic minerals and their compounds (elementary substance) in Paleogene evaporate proves that diversified metallic minerals exist in evaporate. The source of metallic ions in the Neogene evaporate series shows that evaporate could provide materials for late-stage metallic mineralization.     

米粒状白云石及其出溶成因

米粒状白云石产于奥陶系近岸碳酸盐岩序列的云斑(生屑)泥晶灰岩、砾屑灰岩和叠层石灰岩中,在虫孔和砾屑粒间最为富集,外形为伸长菱面体,薄片中呈米粒状.晶体无环带或雾心,但内部含大量呈规则分布的细小方解石“包体”,结晶C轴垂直于晶体延长方向.与背景方解石泥晶相比,Fe、Mn、Na和Ba含量较高,Sr含量较低.C轴的固定取向排除了石膏假晶的可能,也不是普通白云石化或交代钡白云石的结果,推测是沉淀镁方解石重结晶后在压力增加的条件下经出溶作用形成.     

Deep-water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

The Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault-bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north-western Red Sea has cored, below a massive salt unit, an anhydrite-bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca-sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine-grained laminated sediments to coarse-grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken-wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom-rich oozes formed in marine, shallow-water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca-sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope-foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine-grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.     

Earthquake-induced resedimentation in the Badenian (middle Miocene) gypsum of southern Poland

In the Badenian (middle Miocene) basin of the Carpathian foreland of southern Poland, gypsum breccias occur associated with laminated gypsum deposits. These breccias consist of large clasts of gypsum, carbonates, marls and clay chips of variable size embedded in a gypsarenitic matrix. Constituent gypsum grains and clasts commonly appear to be mechanically abraded and chemically corroded crystals or fragments of selenitic, laminated and alabastrine gypsum. Gypsorudites are commonly accompanied by laminated gypsarenites and gypsolutites which show graded bedding; a vertical sequence of graded gypsum beds showing Bouma sequences may be recognized in borehole sections. Microfolding is common within the folded laminated gypsum, and is closely associated with expressions of extensional strain. Both are accompanied by pervasive microfaulting, suggesting a semi-coherent downslope mass movement. The stratiform geometry of the breccias, together with the intensity of slumping relatively independent of the palaeoslope, suggest earthquake shocks as the initial, main cause. Gypsum deposits form a constant, laterally extensive sequence of different lithofacies. The occurrence of the same lithologies and shallowing-up cycles over a wide area reflects thrusting of the Carpathians over the Carpathian foredeep. Local tectonism has also played a significant role. The tectonic framework favoured activation of dip-slip faults promoting shallow-focus earthquakes. These in turn resulted in the resedimentation of gypsum by slumps, debris flows and turbidites. A similar basinward resedimentation of clastic material by gravity flows initiated by fault-induced earthquakes could be of great importance in the foreland geological setting, and may explain some phenomena observed in other evaporite formations from different geological settings, especially of rift type.     

Speleothems from Mawsmai and Krem Phyllut caves,Meghalaya, India: some evidences on biogenic activities

The Mawsmai cave and Krem Phyllut caves, East Khasi hills, Meghalaya, India has so far not yet attracted the attention of geomicrobiologists. Observations and hypotheses on the possible influence of identified microorganisms for speleothem formations in Meghalaya are reported for the first time. XRD studies identified calcite in speleothems and gypsum in cave wall deposits as the dominant minerals. SEM-EDAX showed interesting microfabric features showing strong resemblance with fossilised bacteria, calcified filaments, needle calcite and numerous nano scale calcite crystals, highly weathered and disintegrated crystals of calcite, that point towards a significant microbial influence in its genesis. Thin section petrography showed laminated stromatolitic features. The microorganisms identified by conventional isolation and further evaluation of isolates by molecular techniques include Bacillus cereus, Bacillus mycoides, Bacillus licheniformis, Micrococcus luteus, and Actinomycetes. Microscopic observations also showed unidentifiable cocci and four unidentifiable strains of CaSO₄ (gypsum) precipitating bacteria. Experimental studies confirmed that these bacteria are able to precipitate calcium minerals (calcite, gypsum, minor amounts of dolomite) in the laboratory. These results allow us to postulate that species like these may contribute to active biogenic influence in the cave formations at Meghalaya.     

Authigenic pyrite and gypsum in South West African continental slope sediments

An unusual association of authigenic pyrite and authigenic gypsum has been found in silty clays recovered from the South West African continental slope. Nannofossil content suggests that the sediments are Upper Miocene-Lower Pliocene in age. Pyrite occurs as (1) granular masses, (2) ‘worm’tubes, (3) foraminiferal infillings, and (4) framboids. Gypsum occurs as euhedral single or twinned crystals of selenite. ‘Worm’tubes and foraminiferal infillings of pyrite are partially or completely enclosed in some gypsum crystals. Electron microprobe analyses show a relatively high concentration of manganese in both the granular masses and tubes. Present-day waters off this coast are dominated by the upwelling of cold, nutrient-enriched waters (the Benguela Current). These rich waters support an enormous population of plankton. Death and decomposition of these plankton consume oxygen, thereby creating a belt of anaerobic sediments close to shore. An Upper Miocene-Lower Pliocene regression (Dingle & Scrutton, 1974) lowered sea level and shifted an older analogue of this upwelling zone seaward, eventually establishing an anaerobic environment in places on the present continental slope. Anaerobic bacteria thrived in these conditions. They reduced SO₄ dissolved in sea water, initiating the formation of H₂S. The H₂S reacted with iron minerals present in the sediment to form FeS. Addition of elemental sulphur produced pyrite. This strongly reducing, low pH, environment became saturated with calcium obtained by the dissolution of planktonic calcareous organisms. Gypsum was precipitated once the product of the concentrations of dissolved calcium and SO₄ exceeded the gypsum solubility product.     

Gypsum facies transitions in basin-marginal evaporites: middle Miocene (Badenian) of west Ukraine

In the middle Miocene Badenian gypsum basin of the Carpathian Foredeep, west Ukraine, three main zones of gypsum development occur in the peripheral parts of the basin. Zone I consists entirely of stromatolitic gypsum formed in a nearshore zone. Zone II is located more basinward and is characterized by stromatolitic gypsum in the lower part of the section, overlain by a sabre gypsum unit. Zone III occurs in still more basinward areas and is characterized by giant gypsum intergrowths (or secondary nodular gypsum pseudomorphs of these) in the lowermost part, overlain by stromatolitic gypsum, sabre gypsum and then by clastic gypsum units. Correlation between these facies and zones has been achieved using lithological marker beds and surfaces. Of particular importance for correlation is a characteristic marker bed (usually 20–40 cm thick) of cryptocrystalline massive gypsum occurring in zones II and III. The marker was not distinguished in zone I, possibly because this bed is older than the entire gypsum section of that zone. These new results strongly suggest that the deposition of giant gypsum intergrowth facies and stromatolitic gypsum facies was coeval. In some sections of zones I and II, limestone intercalations have been recorded within the upper part of the gypsum sections. Considerable scatter of the δ¹⁸O and δ¹³C values of these limestones indicates variable diagenetic overprints of marine carbonates, but a marine provenance of the limestones is confirmed by microfacies analysis. Some of the limestones are coeval with an intercalation of gypsarenitic, mostly laminated gypsum occurring in the sabre gypsum unit of zones II and III. Badenian gypsum formed in extremely shallow‐water to subaerial environments on broad, very low relief areas of negligible brine depth, which could be affected by rapid transgressions. Stable isotope (δ³⁴S, δ¹⁸O) studies of the gypsum demonstrate that the sulphate was of sea‐water origin or was derived from dissolution of Miocene marine evaporites. Investigations of individual inclusions in the gypsum indicate decreased water salinity when compared with modern marine‐derived, calcium sulphate‐saturated water. Groundwater influences are indicated by high calcium sulphate contents of the brines in the evaporite basin. The chemical composition of Badenian waters was thus a mixture of relic sea water (depleted in NaCl), groundwater (enriched in calcium sulphate) and surface run‐off.     

Gypsum–anhydrites in 1.9 Ga Vempalle Formation,Cuddapah basin,India: A note on the Palaeoproterozoic environment and diagenetic condition

The Cuddapah basin consists of generally well-preserved Palaeoproterozoic–Neoproterozoic sedimentary and associated volcanic rocks. The detailed lithological studies of sedimentary rocks of Vempalle Formation from the narrow linear belt of 15 km, in the southern margin, show the occurrence of layered gypsum in the cherty dolostone–shale intercalated facies, red shale and phosphatic dolostone facies of the Vempalle Formation. The petromineralogical studies reveal that gypsum is in close association with anhydrites. Microscopically, three different types of gypsum and anhydrite are identified, viz., lath-shaped, equant-shaped and anhedral-shaped grains. The equant variety corresponds to a granular gypsum, whereas the anhedral grains of gypsum exist as the granular and fibrous variety as seen in the hand specimen. The presence of gypsum/anhydrite has been confirmed by the petromineralogical, X-ray diffraction and chemical analytical data. The phosphatic dolostone is the host rock for stratabound type of uranium deposit at Tummalapalle, Cuddapah district, Andhra Pradesh, which is one of the most unique types of uranium mineralisation in the world. Abundant pseudomorphs of gypsum and anhydrite relicts and discontinuous gypsum layers within these dolostones, nodules of chert and gypsum indicate the interrelationship between the diagenesis and genesis of uranium mineralisation which indicates the carbonate precipitation in the sulphate-rich hypersaline environments.     

Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia

Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.